The overall objective of this project is to determine whether treatment of acute hyperammonemia with N-carbamyl-L-glutamate (NCG) in propionic acidemia (PA), methylmalonic acidemia (MMA), carbamyl phosphate synthetase 1 deficiency (CPSD) and ornithine transcarbamylase deficiency (OTCD) changes the clinical outcome of disease. The specific aims are: 1. To determine whether NCG treatment of acute hyperammonemia in severe, neonatal-onset PA and MMA improves neurodevelopmental outcome, and whether it is safe. 2. To determine whether NCG treatment of acute hyperammonemia accelerates the resolution of hyperammonemia and clinical recovery in patients with severe PA and MMA and in those with partial CPSD and OTCD, and whether it is safe. 3. To determine whether, in metabolically stable patients, the effect of a 3-day NCG treatment on ureagenesis is predictive of the outcomes observed in Aims 1 and 2. This is a double blind, placebo controlled trial performed in seven children's hospitals across the country. The primary outcome measure for Aim 1 is neurocognitive development assessed using age-appropriate specific neurodevelopmental instruments. Primary outcomes of Aim 2 include the effect of NCG on the pace of resolution of hyperammonemia and its clinical symptoms and the length of hospitalization. The Aim 3 outcome is an assessment of the correlation between the short-term effect of NCG on ureagenesis rate and the clinical response to NCG long term and in acute clinical settings as determined by Aims 1 and 2. Our hypothesis is that NCG will improve clinical outcome in these disorders. These studies are designed to provide scientific evidence for NCG efficacy and safety which can be used to expand the indication of this drug, leading to FDA approval. PUBLIC HEALTH RELEVANCE: Elevated ammonia levels in the blood can cause severe brain damage, developmental disabilities and can be fatal. N-acetylglutamate (NAG) is a small molecule in the liver that is essential for the urea cycle, keeping blood ammonia levels below levels that are toxic to the brain. We now have strong preliminary evidence that N- carbamylglutamate (Carbaglu), a chemical that is very similar to NAG, but that, unlike NAG, is not broken down in the body, can mimic the effect of NAG to decrease ammonia levels in patients with a number of inherited metabolic diseases. This project will investigate whether Carbaglu can improve the clinical outcome and reduce or prevent brain damage from ammonia in patients with four different genetic diseases associated with high ammonia levels. If the results confirm our hypothesis, these patients can be treated with Carbaglu to keep their ammonia level at normal or close to normal levels, protecting them from brain damage.